Protective relay devices are necessary elements of an electrical power distribution system, as they provide a variety of protection and control functions. Protective relays monitor conditions in the power distribution system and operate circuit breakers in response to the detection of adverse conditions, thereby protecting various segments and components of the power distribution system from damage.
Older protective relays were analog devices, which have been largely replaced by digital protective relays. Digital protective relays include digital processing circuitry which can be programmed to provide a wider variety of protection and control functions than were available with analog relay devices.
Typically, digital protective relays include relay processing circuitry to perform various protective control functions. The relay processing circuitry can be implemented by various configurations of input/output (I/O) circuits, current transformers, voltage transformers, etc. These configurations are generally fixed within the protective relay unit, making it relatively difficult and time-consuming to repair or replace the relay processing circuitry or portions thereof.
U.S. Pat. No. 4,742,428 to Conrad discloses a protective relay which includes an operating unit that can be withdrawn from an insulating housing through an opening in the front of the housing. The withdrawability of the protective relay circuitry allows for routine testing, maintenance, and replacement of the internal relay circuitry with minimal disruption of the power distribution system. However, the Conrad protective relay includes otherwise conventional relay circuitry which is fixed in location within the withdrawable operating unit.
It would be desirable for a protective relay device to allow relatively easy replacement or repair of relay processing circuitry.